Electron paramagnetic resonance (EPR) allows spectroscopic analysis of substances based on physical concepts analogous to those used in nuclear magnetic resonance (NMR). While NMR allows analysis of substances containing nuclides with non-zero spin, EPR is only applicable to substances containing chemical agents that possess at least one unpaired electron. NMR proves particularly useful in the analysis of substances comprising hydrogen atoms, which are abundantly present in water and hydrocarbons. Furthermore, Magnetic Resonance Imaging (MRI), an imaging technique based on NMR, is a valuable tool in medical diagnosis, due to the subtle contrasts caused by water density and complex spin-spin and spin-lattice interactions in different tissues.
EPR, on the other hand, has found less application in the past because all electrons in most stable chemical compounds are paired. However, the strength of EPR lies in its high specificity. EPR can readily be used for detection and imaging of free radicals in tissues, but the development of specific spin-labeled biological tracer molecules has spawned opportunities for the usage of EPR, and particularly the usage of EPR-based imaging techniques, for analysis of diverse physiological functions in biology and medicine. This opens the way for new tracers, specific to biological mechanisms that can't be studied by conventional means, and for alternatives to tracers used in nuclear medicine, without the implied radiation exposure caused by radionuclides.
EPR typically uses DC magnetic fields of 5 mT to 1.25 T or higher to cause magnetic polarization of particles with non-zero electron spin. Narrow-band radio-frequent waves are used to disturb the magnetization and cause resonance. The frequency at which resonance occurs, referred to as the Larmor precession frequency, is dependent on the applied magnetic field strength and specific material properties, and can range from 200 MHz for low field strengths to 35 GHz or higher for strong fields. The low-field (<30 mT) low-frequency (<1 GHz) region is particularly of interest for applications in biology and medicine because of diminished dielectric loss in tissues.
In Journal of Magnetic Resonance 154 (2002) 287, Yamada, Murugesan et al. compare the two commonly used techniques for EPR spectrometry and imaging, namely Continuous Wave EPR (CW-EPR) and pulsed EPR.
CW-EPR is characterized by the use of prolonged RF excitation wave exposure and a resonant cavity or compound resonator. An impedance match between resonator and RF source allows indirect detection of EPR. When electron resonance occurs, the impedance of the resonator alters, and the EPR signal can be inferred from changes in signal absorption. Typically, magnetic field sweeps in combination with constant frequency RF excitation are used to obtain EPR spectra. CW-EPR may require longer acquisition times and may hence be more susceptible to motion artifacts. Imaging techniques based on CW-EPR offer relatively low resolution and low sensitivity and can exhibit diverse artifacts related to magnetic field modulation, power saturation and motion.
Pulsed EPR, on the other hand, offers higher sensitivity, higher resolution, less artifacts and lower acquisition times. In pulsed EPR, a short, intense RF excitation pulse is used to simultaneously excite spins in a narrow frequency band, followed by detection of the impulse response. The resonator requirements are quite different for pulsed EPR compared to CW-EPR. The resonator for use in pulsed EPR should possess efficient RF power to magnetic field conversion characteristics and a recovery time which is shorter than the response time of the EPR signal. These requirements are mutually conflicting, and limit the suitability of pulsed EPR to signals emitted by paramagnetic particles with narrow linewidths, and therefore long relaxation times, since broad linewidth particles would require an extremely short resonator recovery time. However, many commonly used spin probes, or other feasible spin probes fulfilling the non-zero spin condition, possess broad linewidths.